The invention relates generally to computer system input devices and, more particularly, to a trackpad input device having low electromagnetic interference characteristics.
A trackpad is a touch-sensing planar digitizer input device used instead of, or in conjunction with, a mouse, trackball or joy stick. During use, an operator places a finger on the trackpad and moves the finger along the touch-sensing planar surface. The trackpad detects the movement of the finger and in response provides motion signals to a computer. Typically, a trackpad software program converts the detected movement pattern into specific cursor control signals (e.g., location, direction and magnitude of motion).
There are two common types of trackpad sensor devices: resistive and capacitive. A resistive trackpad sensor is a mechanical sensor that uses two layers of material that are typically separated by air. Pressure from a finger pushes the top layer (generally a thin, clear polyester film) so that it touches the bottom layer (generally glass). The voltage at the contact point is measured and the finger's location is computed and transmitted to a host computer system. After the finger is removed, the top layer “bounces back” to its original configuration. A capacitive touchpad sensor, in contrast, is a solid-state sensor made using printed circuit board (“PCB”), flex circuit of glass substrate technology. A finger on, or in close proximity to, a top grid of conductive traces changes the capacitive coupling between adjacent traces. This change in capacitance can be measured and finger position computed.
Like virtually all electronic circuits, the computer system (e.g., notebook or other personal computer, workstation or server computer) in which a trackpad (i.e., a trackpad sensor and its associated electronics) resides generates electromagnetic interference (“EMI”). It will be recognized that such interference can cause damage or a malfunction in other computer system devices or circuits. In the past, the problem of computer EMI has generally been addressed by enclosing the computing environment (e.g., the computer system's motherboard) in a shielded volume (typically referred to as a Faraday cage) and placing the trackpad outside this volume. Often, because the trackpad must be electrically coupled to the computer system, the trackpad itself is partially enclosed in a second shielded container.
Referring to FIG. 1, for example, trackpad sensor 100 and associated circuitry 105 (collectively, the trackpad) is often partially enclosed in shielded (e.g., metal) well 110. Because volume is a critical resource in portable computers, well 110 is as small as possible—often requiring hole 115 be cut therein to permit trackpad circuitry 105 to protrude. This, unfortunately, permits EMI to escape well 110. To combat this problem, circuitry 105 and hole 115 may be covered with a copper foil pad or blanket.
Referring to FIGS. 2A and 2B, another prior art approach to combating trackpad EMI is to provide a single copper pour on the trackpad sensor's PCB. In one embodiment, trackpad sensor PCB 200 has metal layer 205 applied to its bottom surface (i.e., away from the surface providing touch-sensing). In another embodiment, trackpad sensor PCB 210 has internal metal layer 215 beneath its touch-sensing elements. A drawback to the shielding approaches of FIGS. 2A and 2B is that metal layers 205 and 215 introduce large parasitic capacitances. As one of ordinary skill in the art will recognize, such parasitic capacitances decrease the touchpad's operational signal-to-noise ratio which, in turn, can lead to lower performance and/or require additional design responses to mitigate.
It would be beneficial, therefore, to provide a touchpad sensor with low EMI characteristics without introducing large unwanted capacitances or the need to employ additional design techniques to offset the side-effects of the reduced EMI design.